#include<iostream>
#include<string>
#include<cmath>


using namespace std;
string s;

void todo(int low,int high){
    int mid=(low+high)/2;

    if(low!=high){
        todo(low,mid);
        todo(mid+1,high);
    }
    int i,a=0,b=0;
    for(i=low;i<=high;i++){
        if(s[i]=='0') a++;
        else b++;
    }
     if (a&&b) printf("F");
    else if (a) printf("B");
    else printf("I");

}


//1I 0B 01F
int main(){
    int n;

    cin>>n;
    cin>>s;
    todo(0,pow(2,n)-1);

    return 0;
}
